This invention is related to wireless devices, and in particular to automatic gain control for a packet based radio receiver as might be used in a wireless data network.
Wireless technology is well known and widely used. Networks, such as local area networks are also well known and commonly used. Recently, there has been a lot of effort to implement wireless data networks, in particular wireless local area networks (WLANs). There is a desire to make these networks faster and faster. Prior art wireless systems have in general been limited to rather modest data rates. Such small bandwidth can be aggravating in modern Internet uses.
John D. O'Sullivan, et al., describe portable computer wireless local area network devices that operate in excess of 10 GHz in U.S. Pat. No. 5,487,069, issued Jan. 23, 1996, (herein “O'Sullivan '069”). One object of such devices is to allow portable computer users to access the enterprise's LAN untethered and from any location in several buildings on a campus. A method of converting data into symbols that are used to modulate the radio carrier is offered by O'Sullivan '069 to overcome the problems inherent in spread spectrum systems. The use of symbols establishes many parallel sub-channels that each has modulation periods much longer that any multipath delays that might confuse demodulation. Such Patent is incorporated herein by reference. In effect, O'Sullivan '069 describes the basic coded orthogonal frequency division multiplexing (COFDM) called for in the recently adopted IEEE-802.11a wireless LAN standard.
Carrier frequencies in the ultra-high frequency (UHF) radio bands and above can naturally carry very high modulation rates, so more data bandwidth is inherently available.
Automatic gain control (AGC) for radio receivers is well known and widely used. AGC in general is straightforward for systems that receive continually transmitted signals. Communication in a wireless data network is packet-by-packet (“packetized”). Furthermore, packets might be arriving simultaneously from several radio transmitters, so each set of packets from a particular transmitter requires its own gain setting. Furthermore, a wireless receiver does not know when packets start. Furthermore, the high data rates of newer wireless data networks lead to problems when trying to include AGC in receivers.
The IEEE-802.11a burst transmission begins with a two-part preamble, e.g., a short preamble part and a long-preamble part. The exact start of the burst time (SOP) is important to know not only for the purpose of AGC, but also, for example in order to correctly decode the preambles and to carry out the receiver's subsequent demodulation process. There is thus a need to determine SOP quickly in an environment where the carrier frequency and code phase are uncertain. There is also a need to determine the correct gain setting over and over again and relatively quickly; there typically is not much time available for SOP and gain determination. Then there is a need to quickly set the gains of one or more of the stages in the receiver correctly.
Radios-on-a-chip and accompanying modulator-demodulators (modems) on a chip are now being promoted by several companies, e.g., Atheros Communications (Sunnyvale, Calif.) which markets its AR5000 chipset, as does the assignee of the present invention. Such chips put complete 5.15-5.35 GHz transceivers on a chip and complete COFDM modems on another chip, and these chipsets need only few external filters, a transmit/receive switch and a crystal to operate.
Many applications include stations that are battery operated, and for such applications, long battery life is highly important. Thus there is a desire to operate a modem chip at very low power levels.
Thus there is a need not only for rapid start-of-packet (SOP) detection and rapid automatic gain control. There also is a need to carry this out with relatively little power. In particular, there is a need for a method and circuit that achieves SOP detection and AGC compliant with the IEEE 802.11a and similar standards.
The need for rapid AGC has been acknowledged by others for other applications. For example, U.S. Pat. No. 5,524,009 to Tuutijarvi, et al., issued Jun. 4, 1996, deals with mobile communication, e.g., cellular communication, and in particular, with a method and means in such communication systems for providing a shorter handoff time at a mobile station, and for providing an improved gain control value for the receiver at a mobile station. U.S. Pat. No. 5,524,009 describes an AGC method that uses received signal strength determination over a period of about 5 ms. Packets that conform to the IEEE 801.11b standard for data communication in the 2 GHz range at up to 10 Mbits/second, in contrast, include a 64 μs preamble. The IEEE 802.11a standard for data communication in the 5 GHz range at up to 54 Mbits/second, in contrast, has packets with a short preamble and a long preamble that are each only 8 μs long, and AGC should be complete with about half the short symbols remaining. Thus, by rapid IEEE 802.11a AGC is meant AGC in sufficient time to meet the IEEE 802.11a standard.
Prior art AGC techniques that are required to be very fast—a time measured by microseconds or tens of microseconds—have used relatively inaccurate methods. One, for example, starts with a high gain value that causes the analog-to-digital converter for the received signal to overload, and decrease the gain or gains until no overload occurs. This is not only relatively inaccurate, but also requires the ADCs to operate, drawing a lot of power, and requires the ADCs to be overloaded.
One of the goals of AGC in a system is to adjust the gains of the radios to provide maximum dynamic range at the inputs of the analog-to-digital converters. The requirements are quite stringent for a system that conforms to the IEEE 802.11a standard. For example, if the received signal has a level between −62 dBm and −82 dBm at the receiver input, the SOP detection needs to occur within 4 μs with a probability of at least 90%.
Thus there is a need for a relatively accurate, relatively fast, and relatively low power SOP detection method, in particular, one conforming to the IEEE 802.11a and similar standards.
For more information on the IEEE 802.11 and IEEE 802.11a standards, see: ANSI/IEEE Std 802.11, 1999 Edition (ISO/IEC 8802-11:1999) Local and metropolitan area networks—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, and IEEE Std 802.11a-1999 [ISO/IEC 8802-11:1999/Amd 1:2000(E)] (Supplement to IEEE Std 802.11, 1999 Edition) Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band. The standards are available on the Internet at several locations, including from the IEEE (www.IEEE.org) and in particular at http://grouper.ieee.org/groups/802/11/index.html.